This invention relates generally to sewage pumps, and more specifically to an improved priming system for a vertical shaft centrifugal pump of the type used in sewage pump stations.
Sewage pump stations typically include one or more vertical shaft centrifugal pumps that are usually mounted above the sewage level in the sewage receiving wet well. Since such pumps are not able to induce a vacuum by expulsing air from the pumping chamber, to draw liquid thereinto to fill the chamber, and thereby permit the pump to perform its liquid pumping function, it is necessary that the pumping chamber be primed or filled with a liquid by separate means. Such means typically includes a vacuum priming system for withdrawing air from a vacuum chamber in fluid communication with the pumping chamber. Examples of sewage pumps having such a priming system are disclosed in U.S. Pat. Nos. 3,519,369 and 3,558,012, which patents are commonly assigned to the same assignee as this invention.
The vacuum priming system must have a sensing means to determine when sufficient air is withdrawn from the priming chamber to control the operation of the vacuum priming pump. It has been the heretofore practice to utilize either an electrically conductive type sensor or a mechanical displacement type sensor. The conductive type sensor includes a metal rod or electrode to complete an electrical circuit through a relay when the liquid rises around the electrode. The circuit is from a power source through the metal frame, liquid, electrode, relay and back to the power source. Obviously, if anywhere in this circuit it becomes open or non-conductive, the circuit will not function. It has been found that as the electrode becomes coated with grease and solids from the sewage, it becomes non-conductive. Therefore, it has been necessary to clean the electrode every few weeks to avoid malfunction of the priming system. Malfunction can also be caused by bridging of solids or liquids to the frame. The electrode also inherently requires a non-conductive holder, made of a material such as plastic, which reduces the pressure rating of the pump.
The mechanical displacement type sensor typically utilizes either a float to move a set of electrical contacts between a closed or open position or a float containing a magnet that moves up and down a stationary rod containing reed switches that open or close in response to the position of the float. Both of these systems require a moving part in the sewage area that over a period of time tends to seize up. Accordingly, the mechanical displacement type sensors also require frequent cleaning and maintenance.
There is a need for a sensor means for use in the priming system of a sewage pump requiring no cleaning or other maintenance and yet is reliable in operation. There is further a need for such a sensor means that permits a high pump pressure rating. There is still a further need for such a sensor means that does not present a voltage in the sewage area or on top of the pump as is found in certain conductive type sensors.